Phonograph pick-up circuit



M h 1937. c. M. SINNETT 2 1 68 PHONOGRAPH PICK-UP CIRCUIT Filed Sept. 30, 1955 R m g Patented Mar. 9 1937 UNE'TED STATES PHONOGRAPH. PICK-.UP-"GI'RGUIT Chester M. Sinnett, Westmont, N. .L, assignor to. Radiov Corporation of America, a corporation of. Delaware Application September 30, 1935, Serial No, 42,983

13 Claims.

The present invention relates to phonograph pickup circuits for coupling an, electric pickup device to the audio frequency amplifier circuits of a radio receiving system, and has for its. primary object toprovide an improved control and coupling circuit of this character, which provides for the independent control of. tone and volume of. the signals derived from a pickup device, and independent amplifying means therefor.

A further object of the invention. is to provide a phonograph pickup circuit having improved control means. and coupling arrangement for applying phonograph pickup signals, to an output audio frequency amplifier, in conjunction with the audio frequency channel of a radio receiver,

whereby each is independent of the other.

It is alsoa further object of the present invention to provide a phonograph pickup cont-r01 circuit providing independent volume and tone control means in an input network associated directly with the pickup. device and an improved low frequency response without reducing the low frequency range of operation of the network.

'Ijheinvention will, however, be better understood from the following. description when-considered in connect-ion with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is schematic circuit diagram of a phonograph pickup control circuit embodying the invention; and

Fig.2:is a series of curves illustrating a feature of. the operation thereof.

Referring to Fig. 1, 5 is a phonograph pickup device which is provided with a step-up transformer 1 having a primary winding 9 and a secondary winding H. The primary winding is provided with a grounded tap I3 and outer terminals I5 and [6. The latter are for a microphone connection; (not shown). The phonograph pickup. is connected with the terminals [3 and I5 which includes a portion of the primary winding adapted to match the impedance of the pickup device.

The secondary of the transformer is of high impedance and is connected with a volume and tone control network, the high and low potential leads of which are indicated at H and I9 respectively, the latter being grounded as indicated. at 2i.

The volume control portion of. the network comprises a potentiometer resistor 23 connected in shunt relation with the transformer secondary; between the leads I? and i9, and having an output variable tap 25. The potentiometer device 23 is further tapped along its. length asJinE- dicated at 21, and is provided with shunt tone compensating circuits 29 and 305150 provide. a change in the frequency characteristic of the pickup signalsas the volume. is. varied;

Voluniecontrol means of; this character is described in the patent to Jacobs; No. 1,938,256. The purpose of this; control is to. compensate for the lack of response of the ear tosounds at low. volume in the low and: high: audio. frequency ranges and will: hereinafter-be ref-erred. to as. a compensated volume control means. Further description. is believedto.be-unnecessary. I

The network is-.further: shunted by high frequency tone control means. comprising a series variable resistor 3.1: and capacitor- 33 providing a series shunt circuit 35 across the transformer secondary Hg. The variation. ofthe resistor 3l' serves. to vary the effectiyenessjofthe capacitor 33 as a shunt means for high. frequency signals, a reduction of;- the. resistance causing the high frequency tone to be reduced; The circuit is completed to the lead l9 througha lead 31 also connected to the ground 2 l and forming, with the lead, 19, ,a DDIlZiOIl'Of the low potential: side of the network.

A low'frequency tone control-means is also provided in the network in; association with the volume control and high. frequency tone control means, and comprises a capacitor 39 connected with the-lead H, a resistor 4|. and aninductor 43 connected with the 1ead=3 l, the threev elements being in series relationin the order named between the high potential side and the low potential side of the network.

The capacitor and, inductor 43; provide a series tuned circuit, substantially in shunt to the secondary of, the transformer, andtuned to provide a peak in the audio frequency response of the network of the system in the low audio frequency range, thereby to increasethe bassresponse of the system.

The capacitor. 39; is provided with a shunt circuit connection comprising a lead 435;; a variable resistor 41 and a second capacitor-"43', theeffectiveness of which capacitor is varied by means of the resistor 41, thereby to increase the total-cainafter be "considered in connection with the op-.

eration.

In a combinatiq tem wherein the audio frequency amplifier of. the radio receiver system is utilized for both phonon lradio and phonograph sys graph and radio operation, it is desirable to provide means for switching from phonograph to radio operation while providing predetermined adjustment of volume and tone from the phonograph circuit, since the latter involves tone and volume requirements different from the adjusted condition of the radio for proper response to radio signals.

Heretofore, this has not been possible because of the circuit arrangement in such combination systems. In accordance with the invention, however, only the output stage of the radio system is utilized and the phonograph pickup circuit includes the volume and tone control means directly in the network associated with the pickup device and entirely independent of the radio circuit. Furthermore, the phonograph amplifier system is separate from the radio audio frequency amplifier system and is coupled to the output stage in a manner which permits a simplified switching from phonograph to radio operation.

In the drawing, the output stage of a radio receiving system is represented by the amplifier device 5I having signal input or control grids 53 and 55 arranged in balanced or push-pull relation to each other in connection with the secondary 5! of a phonograph input transformer 59. The transformer secondary is center tapped as indicated at 6| for push-pull operation and the terminals 63 are connected with the control grids 53 and 55.

The terminals 63 are also connected through a pair of switches 65 and coupling capacitors 61, with the terminals 69 of the secondary H of an audio frequency input transformer I3 for the radio receiving system (not shown). The primary 15 of the transformer I3 is connected with terminals 'I'I providing the radio output signal as indicated. The secondary is center tapped, as indicated at 19 and connected to the center tap SI of the phonograph input transformer 59 through ground connections 8I.

The phonograph volume and tone control network is connected with the input primary 83 of the phonograph input transformer through a two stage audio frequency amplifier comprising a first stage amplifier electric discharge device 85 and a second stage amplifier electric discharge device 81.

The first stage device is a multiple element tube having an inner control grid 89 coupled to the volume control contact 25 through a coupling capacitor 9I and grid-leak bias connection 93, and a shielded lead 94, the shield 95 being connected to ground as indicated at 96.

The tube 85 is also provided with a volume expander control grid 91 andan output anode 99. The operation of the device 85 as a volume range expander stage is described in my copending application Serial No. 42,982, filed September 30, 1935, for Dynamic amplifier, and assigned to the same assignee as this application. The operation of the stage is such that as the signal strength increases from the phonograph pickup, the amplification factor of the device 85 is then increased to give volume range expansion as well as amplification.

The output signals are derived from the anode 99 and are applied to the second stage device 87 through an output lead IIII, in which is located an anode output resistor I03 and coupling is derived through a coupling capacitor I 95 connected to the control grid I91 of the device 81. The

control grid II" is provided with a grid leak connection I09 to ground and to a self bias resistor III in circuit with the cathode H3. The self bias resistor is provided with a bypass capacitor H5 to ground.

A second grid III and an output anode H9 are connected together as an anode electrode, in circuit with the primary winding 83 and an anode current limiting resistor I2I. A switch I23 is provided for opening the anode circuit upon closure of the switches 65 and is connected therewith as indicated by the dotted connection I25 for simultaneous operation therewith as described.

It will be noted that the devices 85 and 81 are of the metal shield type having an outer metal shield connected to ground as indicated at I21 and may be of the type known on the commercial market as the RCA 6L? and 6C5 respectively.

It will be noted that for radio operation, the plate circuit of the second phonograph amplifier stage may be broken by the operation of the switch I23, thereby removing the plate load of the second stage and connecting the secondary II of the radio input transformer substantially in parallel with the secondary 51 through the coupling capacitors 67 which are of relatively low impedance to audio frequency currents.

For phonograph operation, where high fidelity operation is desired, the switches 65 are opened thereby disconnecting the secondary 'II and constituting the output stage as a third stage directly in the phonograph amplifier, without the shunt load of the secondary II.

The use of the secondary 51 in shunt with the secondary II of the radio input transformer is of value in the reception of radio signals to increase the bass frequency response, thereby improving the lower frequency response from signal sources deficient in low audio frequency tone.

Furthermore, any adjustments of the tuning and volume of a radio receiving system connected to terminals 'I'I does not effect the adjustment of the phonograph system and the change from one to the other is effected by a simple switching operation directly at the output stage.

In the phonograph circuit, the volume and tone control adustments are not only independent of the radio controls but are made in one network, directly in connection with the pickup transformer, whereby the initial signals may be controlled prior to the volume expansion operation and amplification. Furthermore, as such as Y the tone and volume control devices are generally grouped in one place in a phonograph combination apparatus, all of the connections for tone and volume control may be made short to prevent extraneous signals and noise potentials from entering the system at the input thereto. The high potential leads of the network are suitably shielded by metallic grounded shields indicated in the present example at I29.

The tone and volume control network in association with the coupling transformer is arranged to provide an audio frequency bass response which may be varied without reducing the bass response below the normal level of the overall frequency characteristic. In the series resonant circuit 39, M, 43 in shunt with the transformer secondary, resonance is adjusted at a predetermined frequency which in the present example, is approximately 30 cycles. This is shown in Fig. 2, to which attention is now directed, along with Fig. 1.

The normal response level of the network is indicated by the straight portion of the curve I30. By means of the tuned series resonant circuit, the low frequency response is raised as indicated by the peak l3l.

Lil

' ohms.

The position of the peak is adjusted by varying the capacity of the capacitor 39, a lower capacity value causing the peakto shift upwardly in the frequency scale, indicated by the dash and dot line curve I33, while an increase in the capacity at 39 causes the peak to shift in a downward direction as indicated' by the peak I35.

Further increase in the capacity at 39' causes the peak toshift further down in frequency until the peak is belowzero frequency as shown bythe peaks at I31. It will be seen, however, that the audiofrequency bass response in the solid portion of the curves. at I39 and Ml is held above the level of theoverall response characteristic represented by the curve I30.

The-bass response control arrangement, therefore, is such that the bass response characteristic is provided with a peak above the normal level of the response characteristic by a series resonant circuit across the network and in shunt relation to the secondary of the pickup coupling transformer and the tuning of the series resonant circuit is such that as the series capacity is increased the bass response peak may be moved below the audio frequency range while maintaining the response in the audio frequency range in the bass portion above the normal overall response level of the frequency characteristic of the system.

In tone control systems, heretofore known, control of bass tone requires the reduction of the audio frequency response below the normal overall response level characteristic of the system. With the present arrangement, the bass response is maintained to provide substantially fiat response characteristic throughout the lower end of the audio frequency range when the bass response is reduced to a minimum level.

In the present example, it has been found that this effective control may be obtained by utilizing the capacitor 49 in shunt relation to the capacitor 39 to increase the effective capacity in the series resonant circuit as the resistance value of the variable resistor 41 is reduced. The

audio frequency bass response characteristic is, therefore, variable to and between the various points of adjustments as indicated by the curve of Fig. 2 byoperation of the variable series resistor 41 and the initial or maximum bass response dependent upon the initial. capacity. in the series resonant circuit.

In the present example, with an inductance of .25 henries at 43 and a capacitor at 39 of .035 microfarad, the network is resonant to approximately 30 cycles as indicated by the peak I3l in the curves of Fig. 2. The resistor 4! serves to reduce the height of the peak to a desired Value with respect to the normal level of the characteristic curve and, in the present example, is 4700 ohms.

The additional tuning capacitor 49 may be of the order of .5 microfarad and the controlling resistor 41 may have a resistance of 200,000 The high frequency tone control resistor 31 may have a total resistance of 100,000 ohms and the highfrequency tone control capacitor 33 may have a capacity of .015 microfarad. The volume control, potentiometer 23 may have a total resistance of 675,000 ohms.

From the foregoing description it will be seen that the tone and volume may be controlled independently in the input network with effective bass response for all positions of the tone control means and that the control circuit connections are thus grouped with the control devices as is desirable to, reduce hum and noise pickup.

The switching operation is confined to the output stage and provides further bass tone control for radio operation by the shunt secondary con-v nection for the radio input circuit tending to increase or improve the bass response in addi-- tion to the bass adjustment provided by the input network.

I claim as my invention:

1. In a radio-phonograph system, the combination of an audio frequency amplifier output stage, radio phonograph switching means connected with said output stage and comprising radio and phonograph amplifier channel t'ermi-- nating transformers having secondaries connected in parallel forradio reception thereby to increase the bass response of the output stage,

and independent bass response control means forthe phonograph amplifier channel comprising a volume and tone control network including a series resonant circuit including thesecondary of a phonograph pickup coupling transformer and a controlling capacity, and means forvarying said capacity to causesaid circuit to resonate below the effective audio frequency response range of said pickup'circuit.

2. In a radio-phonograph system, the combination of a phonograph pickup input circuit, a

tone and volume control network in said cir cuit, means in said network for controlling the bass frequency response thereof, said means in- I cluding a tuned circuit variable to cause an audio frequency peak within and without the audio frequency range and above the normal frequency response level of said network, an audio frequency output amplifier stage connected with said network to amplify signals therefrom, means for applying amplified radio signals at audio frequency to said output stage, and switching means for controlling said last named means to change the bass response of said output stage when amplified radio signals are applied thereto through said last named means.

' 3. A radio-phonograph control system comprising, in combination, an audio frequency amplifier output stage, phonograph and radio signal input transformers for said stage, means for connecting the secondaries of said transformers in parallel and opening the primary of the phonograph signal input transformer for amplifyingradio signals whereby the frequency response characteristic of the output stage is modified for the amplification of radiosignals, an audio frequency amplifier connected with said phonograph graph signal input transformer for amplifying radio signals whereby the frequency response characteristic of the output. stage is modified for the amplification of radio signals, and meansfor, independently modifying the frequency response characteristic oflphonograph pickupsignals coma prising a volume and tone control network preceding in circuit the phonograph input transformer for said output stage and comprising the secondary of a phonograph pickup coupling transformer having a relatively high impedance, a capacitor and an inductor providing with said secondary a tuned circuit for raising the audio frequency response characteristic in the low frequency range above the normal level of the response characteristic of said network, means for varying the tuning of said circuit to vary the low frequency response characteristic, and means for maintaining said low frequency response characteristic above said normal response level of the network.

5. A phonograph pickup control circuit comprising in combination, an audio frequency coupling transformer having a high impedance secondary winding, a signal transmission network connected directly with said secondary Winding and including volume and tone control means each having independently variable con- ,trol elements, and additional tone control means 6. A phonograph pickup control circuit com-- prising in combination, an audio frequency coupling transformer having a high impedance secondary winding, a signal transmission network connected directly with said. secondary winding and comprising a high potential lead, a low potential lead, tone compensated volume control means and high frequency tone control means connected between said leads, and low frequency tone control means directly in said network comprising a capacitor, a controlling resistor and an inductor providing a series resonant circuit, a second capacitor connected in parallel relation to said first named capacitor, and a variable resistor in series with said second capacitor to control its effective capacity in shunt with the first named capacitor, said circuit being thereby variably resonant between limits within and below the audio frequency range.

'7. In a phonograph pickup control circuit, the combination with a pickup coupling transformer having a primary winding adapted to be connected with a pickup device and having a high impedance secondary winding, of a tone and volume control network connected with said secondary winding having variable volume and high frequency tone control elements therein and a variable low frequency tone control element comprising a single variable resistor and a series resonant circuit in said network and in shunt with said secondary including a capacity element controllable by said resistor to provide a resonant peak in the audio frequency response of the circuit below the audio frequency range.

3. The combination with an output audio frequency amplifier stage comprising a pair of electric discharge amplifier devices each having a control grid, of a phonograph signal input transformer for said stage having a secondary connected in balanced relation with said control grids, a radio signal input transformer having a secondary connected in parallel with the first named secondary and in balanced relation to said control grids, a phonograph signal amplifier having an output circuit connected with the primary of said phonograph signal input transformer, means for opening said output circuit and completing the second named connection, thereby to amplify radio signals and to modify the low frequency response of the output amplifier stage, a volume and tone control network for independently controlling the tone and volume characteristic of phonograph signals, means for coupling said network to said phonograph signal amplifier, and means in said network for independently controlling the low frequency tone characteristic of phonograph signals.

9. Phonograp'h control system in accordance with claim 8 further characterized by the fact that the independent tone control means for bass response includes a series resonant circuit having a controlling capacity therein for producing a bass response peak below the audio frequency range, and a single variable resistor for controlling the effective capacity.

10. A phonograph pickup circuit for radio phonograph combination apparatus comprising a signal output stage, a phonograph amplifier coupled thereto, means for applying radio output signals directly to said output stage and means for controlling phonograph pickup signals ap-.

plied to said phonograph amplifier independently of the tone characteristic and volume levels of radio signals applied to said output stage, said means comprising a phonograph pickup transformer having a high impedance secondary winding, a volume and tone control network connected with said secondary winding and including with the secondary a resonant circuit having a series capacity of a value to tune said circuit within the low audio frequency range, a second capacity for tuning said circuit below the audio frequency range, and a variable resistor for controlling said last named capacity.

11. The combination with an output amplifier stage for a radio receiving system, of a phonograph pickup circuit, independent amplifier and volume control means for said pickup circuit, means for coupling said amplifier means to said output stage independently of radio signal input thereto, low frequency tone control means in said circuit comprising a series resonant path including a series capacitor, a second capacitor connected in shunt relation thereto, and a variable resistor for controlling the last named connection, and means providing a low frequency tone control connection for the output stage when operating in response to radio signals.

12. In a phonograph pickup circuit, a phonograph pickup coupling transformer having a secondary winding, a tone and volume control network connected therewith providing a predetermined level frequency response characteristic, tone control means connected in said network providing a low frequency resonant peak in said characteristic, said last named means comprising a series resonant circuit having a tuning capacitor and a tuning inductance, and variable means for increasing the effective capacity of said series capacitor to cause said peak to fall below an effective low audio frequency range without reducing the normal level of the response characteristic.

13. A phonograph signal amplifier comprising a phonograph input transformer, a volume control network connected directly with the secondary thereof, a volume expander amplifier connected with said network, an audio frequency output stage coupled to said volume expander stage, means for opening the coupling connection with said output stage, means simultaneously controllable therewith for applying radio signals to said output stage in parallel with said coupling means to effect a change in bass response of the amplifier, and. independent bass control means in said network for further controlling the bass response of the amplifier when operating in response to phonograph signals.

CHESTER M. SINNETI. 

